Very long optical fiber transmission paths, such as those employed in undersea or transcontinental terrestrial lightwave transmission systems, are subject to decreased performance due to a host of impairments that accumulate along the length of the optical fiber in the transmission path. The source of these impairments within a single data channel includes amplified spontaneous emission (ASE) noise generated in Erbium-Doped Fiber-Amplifiers (EDFAs), nonlinear effects caused by dependence of the single-mode fiber's index on the intensity of the light propagating through it, and chromatic dispersion which causes different optical frequencies to travel at different group velocities. In addition, for wavelength division multiplexed (WDM) systems, where several optical channels are on the same fiber, crosstalk between channels caused by the fiber's nonlinear index can be problematic.
Distortions of the received waveform are influenced by design of the transmission line, as well as the shape of the transmitted pulses. Known long-haul systems have been implemented using On-Off-Keying (OOK), wherein the transmitted pulse is turned on and off with the ones and zeros of a data bit stream. On-Off-Keying may be implemented in a variety of well-known formats, such as Return-to-Zero (RZ), Non-Return to Zero (NRZ) and Chirped-Return-to-Zero (CRZ) formats. Generally, in a RZ format the transmitted optical pulses do not occupy the entire bit period and return to zero between adjacent bits, whereas in a NRZ format the optical pulses have a constant value characteristic when consecutive binary ones are sent. In a chirped format, such as CRZ, a bit synchronous sinusoidal phase modulation is imparted to the transmitted pulses.
Phase Shift Keying (PSK) is another modulation method known to those of ordinary skill in the art. In PSK modulation ones and zeros are identified by phase differences or transitions in the optical carrier. PSK may be implemented by turning the transmitter on with a first phase to indicate a one and then with a second phase to indicate a zero. In a differential phase-shift-keying (DPSK) format, the optical intensity of the signal may be held constant, while ones and zeros are indicated by differential phase transitions. DPSK modulation formats include RZ-DPSK, wherein a return-to-zero amplitude modulation is imparted to a DPSK signal, and CRZ-DPSK.
It has been recognized that the RZ-DPSK modulation format has particular advantages over other formats in WDM long-haul optical systems. For example, compared to OOK, RZ-DPSK modulation provides a significant reduction in the required optical signal-to-noise (OSNR) for a particular bit error rate (BER). As such, systems for imparting a RZ-DPSK modulation to WDM optical signals have been developed.
A RZ-DPSK modulation may be applied to a signal by imparting a periodic RZ intensity modulation to a continuous wave optical signal, and then imparting a NRZ DPSK data modulation to the intensity modulated signal. The order of the RZ and DPSK modulation may be reversed. Those of ordinary skill in the art will recognize that the RZ intensity modulator and the NRZ DPSK data modulator may be Mach-Zehnder-type optical modulators. Known two-stage Mach-Zehnder type optical modulators conduct modulation in a two-staged manner through serially connected Mach-Zehnder type optical modulators disposed on a single substrate, such as lithium niobate (LN). RZ-DPSK modulation may be generated by selecting appropriate driving voltages and bias points for the serially connected Mach-Zehnder modulators.
Stable and accurate setting of the Mach-Zehnder bias points is necessary to achieve a RZ-DPSK modulated signal that results in optimal system BER. In a RZ-DPSK signal, the modulator imparting the RZ modulation may be biased at the peak of the modulator transfer function, and the modulator imparting the NRZ DPSK modulation may be biased at the null of the transfer function. However, factors including temperature and aging can cause the modulator transfer function to vary, thereby modifying the bias point necessary to achieve optimum performance. Bias control loops have been developed in an attempt to ensure accurate modulator biasing. Known bias control loop configurations have, however, incorporated expensive and/or inefficient configurations.
In addition, optimum performance requires stable and accurate relative alignment between the RZ modulator and the NRZ DPSK modulator. Optimally, the peak amplitude point of the RZ modulation is aligned with the center of the data bits modulated on the signal by the NRZ DPSK modulator. The alignment may be achieved by delaying the RZ modulation relative to the DPSK modulation using an electrical phase shifter/delay circuit coupled to the RZ modulator's drive signal. The optimal setting of the phase shifter to achieve proper alignment can also vary with temperature and aging. In an attempt to address this problem, transmitters have been configured with predetermined alignment settings for various temperatures. This solution, however, requires a complicated and time consuming factory calibration procedure, and does not account for drift in clock-data alignment associated with aging or modulator operating point changes.
There is therefore a need for a system and method for efficiently and reliably controlling the modulator bias points and timing alignment in a RZ-DPSK transmitter.